The invention relates to methods and equipment for synchronization of frames which are transmitted over multiple parallel transmission paths in a telecommunications network.
FIG. 1A illustrates a telecommunications network to which the invention is applicable. The telecommunications network could be e.g. a so-called third generation cellular mobile network, such as UMTS (Universal Mobile Telecommunications System). The telecommunications network comprises a first end node, such as a mobile station MS, and a second end node, such as a Radio Network Controller RNC1, RNC2. The network also comprises several middle nodes, such as Base Stations BS1 to BS4. Information to be sent between the end nodes is formatted as frames and at least some frames are sent between the end nodes via at least two middle nodes. The first middle node via which a connection between the end nodes is established is called a first middle node. The other middle nodes, which are added later to the connection, are called second middle nodes. In cellular network technology, the technique of routing frames via several middle nodes is called macrodiversity, diversity combining or soft handover.
FIG. 1B illustrates a problem associated with the system as shown in FIG. 1A. From this point on, the invention will be described using concepts and terminology from cellular networks, especially UMTS, but it should be kept in mind that the invention is applicable to other telecommunications networks where the middle nodes (the base stations) are not synchronized with each other and/or the end nodes (MS, RNC). In other words, the different nodes do not use a common timing reference and/or frame numbering sequence. However, recent UMTS literature favours the term User Equipment (UE), but in this application, the term Mobile Station (MS) will be used.
According to the current vision of UMTS, some traffic overhead is eliminated by not transmitting frame numbers with the frames (i.e. on a traffic channel) over the radio interface Uu. Instead, in the BS to MS direction, the frame numbers are broadcast to all mobile stations simultaneously, and in the BS to RNC direction, the base stations add the frame numbers in a modulo-p sequence where the currently proposed value for p is 72. In other words, the frame numbers repeat cyclically: 0, 1, . . . , 71, 0, 1, etc. The base stations are not synchronized with each other. Therefore, the frame numbers are relative and, indeed as such, they are meaningless without at least implicit information about the timing reference on which the frame numbers are based. Depending on the point of view, the frame numbers are called MSFN, BS1FN, BS2FN, etc. In FIG. 1B, time advances from top to bottom. At time T0, the Mobile Station MS receives a frame which the MS interprets as frame number N. Because the mobile station's timing reference, i.e. its clock, is the master timing reference, the RNC must have sent this frame sometime before T0. This “sometime” is called Timing Difference Tdiff and it is due to the finite propagation and processing delays between the RNC and the MS. (The concept of the transmission delay is somewhat analogous to the timing advance in the GSM system, but in the GSM, the timing advance of the MS is adjusted.)
At time T0, the MS also sends an uplink frame to the RNC. This uplink frame is also numbered N because the frame numbering is based on the mobile station's timing reference. Under each node (MS, BS1, BS2) is shown the corresponding timing reference, or frame numbering sequence (MSFN, BS1FN, BS2FN). Approximately at time T0+Tdiff, the RNC receives the frame N via two base stations, BS1 and BS2. Because the frame number is not transmitted over the radio interface, and because the base stations use different timing references, the BS1 sends this frame to the RNG as frame N′, whereas BS2 would send the same frame as frame N″. The RNC does not have an intrinsic timing reference.
Although the first problem is most severe in a system using macrodiversity, the underlying problem of BS/RNC synchronization also occurs without macrodiversity.
A second problem is that in some telecommunications systems, such as UMTS, the connection can be reconfigured, e.g. for negotiating a different data rate. Such reconfiguration must take place simultaneously at all nodes involved in the connection.
A third problem is that in systems using encryption, the frame number is often used as a constantly varying cipher key. However, a modulo-72 frame number is too short for reliable ciphering.